(1) Field of the Invention
This invention generally relates to a method and apparatus for establishing a profile of a characteristic property of a medium along an axis and more specifically to a method and apparatus for measuring such properties as sound velocity in a liquid as a function of depth.
(2) Description of the Prior Art
The velocity of sound through a medium, such as sea water, depends upon a number of factors including the temperature, pressure, density and, in the case of sea water, the salinity of the medium. Density is further a function of the temperature, pressure and salinity of the medium as well. When the medium is sea water, the velocity of sound through the medium can vary due to variations in temperature, salinity, and viscosity. In many situations, however, it is important to obtain an accurate profile of sound velocity or temperature through a medium at various positions along an axis such as depth positions. A number of different approaches have been utilized in the prior art.
For example, U.S. Pat. No. 3,441,901 to Cawley et al. discloses a system for measuring sound velocity in water as a function of depth. In accordance with this patent, a freely sinking probe containing a transducer is launched from a moving vessel and connected thereto by a wire link. A sound transmitter aboard the vessel transmits an acoustic pulse into the water. A transducer on the probe receives this pulse and transmits depth and timing signals to instrumentation through the wire link. The sound velocity as a function of a particular depth of the probe is computed. Additional acoustic pulses are transmitted into the water to obtain a profile of sound velocity as a function of depth.
U.S. Pat. No. 4,926,395 to Boegeman et al. also discloses a method and system for measuring sound velocity. The speed of sound in a fluidic medium is determined from the travel time of an acoustical signal for a predetermined distance in the fluidic medium by generating a cyclical reference signal of a predetermined frequency and transmitting a portion of the reference signal through the medium. The transmitted portion of the reference signal is received after traveling a predetermined distance in the fluidic medium. The cycles of the cyclical reference signal are counted during the period of time between the transmitting and receiving of the portion of the reference signal wherein the travel time of the portion of the reference signal is the number of cycle counts divided by the frequency. The speed of the acoustical signal through the fluidic medium is a function of the path length divided by the travel time.
Another technique for estimating the velocity of sound in sea water is to measure the water temperature as a function of depth and to calculate the sound velocity from well established equations. However this approach provides an indirect measurement of sound velocity and the accuracy can be affected by the accuracy with which the temperature can be measured as well as values of measured or assumed depth and salinity. Typically the instrument used in such measurements is a bathythermograph that measures temperature and depth as it sinks slowly through the ocean water. Readings are then transmitted to a ship with a receiving unit that converts this information into a sound velocity profile.
Alternatively, an instrument with precisely located transducers can be used to measure the velocity of sound between them. These instruments, that generally are not expendable, provide a measure of sound velocity at a single depth and usually comprise a towed body. They must be moved slowly up and down in the ocean to generate a sound velocity profile.
In another method, described in May et al., Temperature Sounding by RASS with Wind Profiler Radars: A Preliminary Study, IEEE Trans. on Geoscience and Remote Sensing, Vol.28 No.1 January 1990 (page 19-28), remote measurements of atmospheric temperature profiles are obtained by combining acoustic and radar techniques. Specifically short and long acoustic pulses are suggested with a radar pulse being introduced along the same axis as the acoustic pulse. A coherent radar measures backscattered radiation from fluctuations in atmospheric density induced by acoustic pulses to determine the local speed of sound.
In Vignola et al., Laser Detection of Sound, J. Acoust. Soc. Am.90(3), September 1991 (pages 1275-1286) a differential laser-doppler velocimeter measures the Doppler shift of laser light scattered from colloidal micro particles oscillating under the action of the acoustic field.
U.S. Pat. No. 4,429,994 of Guagliardo et al. discloses a system for remotely determining the velocity of sound in water by means of Brillouin scattering measurements. A pulsed laser irradiates water and the apparatus collects backscattered light and collimates this light for transfer through a Fabry-Perot interferometer to a photomultiplier. A ramp generator drives the interferometer for selecting specific frequencies for analysis, so the instantaneous ramp voltage corresponds to the point in a frequency domain to which the digitized output corresponds. A plurality of readings from multiple laser shots is obtained for storage and computer processing to develop a three-dimensional graphic representation the axes of which are frequency, intensity and depth.
None of the foregoing approaches provides a method or apparatus that provides accurate, rapid, direct measurements of sound velocity. The apparatus of the Cawley patent, bathythermographs and fixed transducer structures require significant times to obtain a full profile. The system disclosed by May et al. is directed primarily to systems for measuring temperature profiles in the air; the absorption of radar signals in water and other liquids precludes the use of this approach in measuring velocities in water. The Vignola et al. article and the Guagliardo et al. patent disclose systems that seem to rely upon the interference between laser light and thermal photons that travel in all directions in the medium. The effect is very weak and the returned signals extend over a wide band. Consequently in order to obtain any accurate measurements it is necessary to record a large number of samples. For example, in the Guagliardo patent over two hundred pulses are required in order to obtain a profile and the time to obtain sample is in the order five seconds. Consequently these, as well as other prior approaches, require significant intervals of time for obtaining appropriate data. Moreover in each of these systems the sound profile is normally obtained indirectly through significant processing operations based upon certain assumptions that are subject to errors.